This invention relates to an improved movement for a small two-hand quartz analog wristwatch, and more particularly relates to such a movement designed to be driven by a Lavet stepping motor with a low starting torque drive.
It is known that a quartz analog wristwatch movement can be designed to use only a minute hand and hour hand with the minute hand mounted on the same shaft as a center wheel and to index the center wheel once per minute with a quartz-controlled rocking motor. Such a construction is shown in U.S. Pat. No. 4,128,992--Egger, et al, issued Dec. 12, 1978. Inasmuch as the rocking motor required two shunt plates for the magnet and placed the gear reduction assembly between the center wheel and the dial, the watch movement tended to be rather thick.
An improved two-hand watch movement for a stepping motor providing a thinner watch construction placed the center wheel just below the dial and inverted the gear reduction assembly in such a manner that the minute hand is disposed on an inner sleeve and the hour hand is disposed on an outer sleeve, both journaled on a fixed center post. This arrangement was unconventional in locating the hour hand above the minute hand with respect to the dial of the watch and greatly reduced the watch thickness. Such a construction is shown in U.S. Pat. No. 4,443,112--Stotz, et al, issued Apr. 17, 1984. Due to the fact that this movement utilized a stepping motor which advanced the rotor only a fraction of a revolution each step, the center wheel required very small teeth and high starting torque at each step, hence, greater power consumption of the energy cell.
The power minimizing advantages of a Lavet motor, with a bi-polar rotor and a one-piece stator construction, wherein the rotor makes a 180 degree rotation each step is known in the prior art. Examples of such motors are shown in U.S. Pat. No. 4,398,107--Fukushima, issued Aug. 9, 1983; in U.S. Pat. No. 4,412,144--Denisov, issued Oct. 25, 1983; and in U.S. Pat. No. 4,335,454--Yamada, issued June 15, 1982.
One of the disadvantages of this type of stepping motor is its low starting torque. One way to overcome this problem is to prevent engagement of either of two pins driven by the rotor until the rotor has moved through a given angular interval, as shown in U.S. Pat. No. 4,084,402--Kitai et al., issued Apr. 18, 1978. Another type of drive for a bi-polar stepping motor using two pins to drive a toothed wheel where the rest position of the pins block the wheel during rest is shown in U.S. Pat. No. 4,357,695--Bachmann, issued Nov. 2, 1982 and in PCT Application, PCT/EP79/0025, filed 30 Mar. 1979 and published 15 Nov. 1979 as WO79/00930.
Lastly, U.S. Pat. No. 4,483,627--Muller, et. al., issued Nov. 20, 1984 discloses a bi-polar Lavet motor with a two-pin drive directly engaging the teeth of a seconds wheel driving a conventional gear train for a three-hand watch. Suggestions in the patent for producing a two-hand watch by altering the construction shown in the patent do not serve to reduce the thickness of the movement with the several layers of gearing shown.
Accordingly, one object of the present invention is to provide an improved thin, two-hand movement for a quartz analog wristwatch.
Another object of the invention is to provide an improved two-hand watch movement suitable for a Lavet bi-polar stepping motor.
Still another object of the invention is to provide an improved two-hand quartz analog stepping motor watch movement with a minimum number of components to reduce its cost.